The B chromosomes of Locusta migratoria. III. Effects on the activity of nucleolar organizer regions

Abstract

Active nucleolus organizing regions (NORs) have been studied in two controlled crosses in the locust Locusta migratoria. NOR activity was detected in all the chromosomes of the standard complement but not in the B chromosomes. On the basis of both the frequency with which they are active and the size of the resulting nucleolus, NORs in this species can be classified in three groups: (i) type I primary NORs that are active in the majority of cells and give rise to large nucleoli; (ii) type II primary NORs that are active in the majority of cells but produce small nucleoli; and (iii) secondary NORS that are active in a minority of cells and yield small nucleoli. The presence of B chromosomes influences the mean activity of some NORs, particularly those giving rise to small nucleoli (groups ii and iii), although different populations yield contradictory results. Our findings indicate that NOR activity in L. migratoria is regulated by genetic and environmental factors, and can be influenced by the presence of B chromosomes which tend to increase the cellular level of NOR activity. An analysis of dependence between NORs for activity demonstrated the existence of a positive dependence between many of the NORs that give rise to small nucleoli. The presence of B chromosomes, however, favoured these enhancement relationships between nonhomologous NORs.Key words: Locusta, B chromosomes, nucleolar organizer.